The present invention relates to a fiber optic gyroscope which employs an optical waveguide formed as an optical integrated circuit for splitting and coupling light.
There has been proposed a fiber optic gyroscope which utilizes, in place of an optical fiber coupler, an optical waveguide formed as an optical integrated circuit, for splitting light from a light source into two for propagation to an optical fiber coil forming a ring interferometer and for coupling two beams of light having propagated through the optical fiber coil for guiding it as interference light to a photodetector. FIG. 1 shows an example of such a fiber optic gyroscope, in which an optical fiber coil 61 is wound in layers around the periphery of a disc-shaped support structure 60 near one side thereof and a light source 62, a photodetector 63 and an optical integrated circuit substrate 70 are mounted on the support structure 60 inside the optical fiber coil 61. The optical integrated circuit substrate 70 is a rectangular electro-optic crystalline plate as of lithium niobate and has on its one surface an optical waveguide 74 formed by diffusing thereinto, for example, titanium in a desired strip pattern. The optical waveguide 74 comprises a trunk 71, a pair of branches 72a and 72b which diverge from the trunk 71 at one end thereof, and another pair of branches 73a and 73b which similarly diverges from the trunk 71 at the other end. The pair of branches 72a and 72b and the trunk 71 constitute a 3dB coupler, whereas the pair of branches 73a and 73b and the trunk 71 also constitute a 3dB coupler. Optical connectors 76 and 77 are provided at opposite ends of the optical integrated circuit substrate 70. The branches 72a and 72b of the optical waveguide 74 are connected to one and the other ends 61a and 61b of the optical fiber coil 61, and the branches 73a and 73b are connected via optical fibers 78 and 79 to the light source 62 and the photodetector 63, respectively. An electric circuit unit 80 is mounted on the other side of the support structure 60.
Though not shown, an electrode pair for phase modulation is provided for either one or both of the branches 72a and 72b of the optical waveguide 74, and the electric circuit unit 80 includes a phase modulation signal generator for supplying a phase modulation signal to the above-mentioned electrode pair and a synchronous detector for synchronous/detection of an output signal from the photodetector 63 by the phase modulation signal to obtain the output of the fiber optic gyroscope. The trunk 71 of the optical waveguide 74 is so formed as to produce a polarization effect.
Light from the light source 62 passes through the optical fiber 78 and is applied from the branch 73a of the optical waveguide 74 via the trunk 71 to the branches 72a and 72b, that is, the light is split into two beams. At this time, either one or both of the beams of light are phase modulated by the application of the above mentioned phase modulation signal to the above-mentioned electrode pairs. As a result, the one light is supplied as left-handed light to the optical fiber coil 61 from its one end 61a and the other light is supplied as right-handed light to the optical fiber coil 61 from the other end 61b. The left-handed light and the right-handed light having propagated through the optical fiber coil 61 are supplied to the trunk 71 of the optical waveguide 74 via its branches 72b and 72a from the other and the one end 61b and 61a of the optical fiber coil 61, respectively, whereby they are coupled together. The resulting interference light is applied from the branch 73b of the optical waveguide 74 via the optical fiber 79 to the photodetector 63, wherein it is converted into an electric signal, which is subjected to synchronous modulation by the above-mentioned phase modulation signal, providing the output of the fiber optic gyroscope. Further, a base 91 is mounted on one side of the support structure 60 and the optical integrated circuit substrate 70 is fixedly mounted directly on the base 91 with an adhesive 92 as shown in FIGS. 2 and 3.
In the conventional fiber optic gyroscope described above, since the optical waveguide 74 formed on the optical integrated circuit substrate 70, which is disposed inside the optical fiber coil 61 serving as the ring interferometer, includes the pair of sufficiently long branches 72a 72b and 73a, 73b which are arranged straight at opposite ends of the sufficiently long trunk 71, the optical integrated circuit substrate 70 is long and the diameter of the loop of the optical fiber coil 61 encompassing it is large accordingly. Thus, the fiber optic gyroscope is inevitably bulky as a whole.
Moreover, since the prior art fiber optic gyroscope mentioned above has the optical integrated circuit substrate 70 joined directly to the base 91 with the adhesive 92, accidental heating from the outside, such as an ambient temperature rise, will develop a high thermal stress in the optical integrated circuit substrate 70 owing to the difference in thermal expansion coefficient between the electro-optic crystal forming the integrated circuit substrate 70 and the base 91 to thermally distort the electro-optic crystal and hence change its refractive index, introducing an error in the output of the fiber optic gyroscope.